Amplifier



Feb. 27, E945. G. Mui-FLY 370,483

AMPLIFIER Fi1ed Aug. 14, 1942 4 sheets-sheet 1 H D m [.1 D O III"- mgm t() HM PUF/ER IM PLI Fl ER GARY M U FFLY Feb, 27, i945. MUFFLY 2,37A33 AMPLIFIER Filed Aug. 14, 1942 4 Sheets-Sheet 2 mam ! S 1 f l N m R .@ARYMUFFLY :FIXED Feb. 27, 1945. G, MUFFLY 2,376,483

AMPLIFIER v Filed Aug. 14, 1942 4 Sheets-Sheet 3 L/f- Xi fwyq wam@

Feb., 2% H945. G. MUFFLY 2,370,483

AMPLIFIER Filed Aug. 14, 1942 4 Sheets-Sheet 4 20N AON 2o- 40M GARY MUFFLY Patented Feb. 27, 1945 l AMPLIFIER Y GaryMuiiiy, Penn TownshimAllegheny County, Pa., assigner to Gulf Research & Development Company, Pittsburgh, Pa.,

. Delaware a corporation of Application August 14, 1942, Serial No. 454,842

i (ci. 179-111) Claims.

This invention relates to improvements in ampliers.

In seismograph prospecting artificially created seismic waves are picked up at one or more positions in the earth by electro-mechanical detectors, and the corresponding electricalwaves developed by the detectors are amplified and recorded. The received waves include a wide range of frequencies, some of which it is desired to record to the exclusion of others.

The requirements to be met in amplifiers for such use are rather severe. A seismograph amplifier should be sensitive to the desired range of frequencies although highlyinsensitive to frequencies outside of that range. It is sometimes difficult to exclude powerful undesired frequencies whichare close tothe desired band. Further, the frequency response function of the amplifier should be adjustable both as to the range of frequencies passed and suppressed and, ,independently of such adjustment, as to the relative sensitivity for the several frequencies. In other` Words, the shape of'frequency response function should, in an ideal'amplier, be readily adjustable.

It is accordingly an object of the present invention to provide an amplifier which closely approaches the ideal above described, being readily adjustable to band width and, independently, to shape of the frequency response curve.

Another object of the invention is the provision of a resistance-tuned amplifier in which variation of the frequency response function is accompanied by varying resistance only.

Other objects are the provision of an amplifier of the type described making use of the inverse feedback principle, and of extraordinary stability as to gain and frequency characteristics even with large variations in battery voltage and in tube aging.

Further objects and advantages of this inven-l tion will be apparent from the following description and drawings'in which:

Fig. 1 is a schematic diagram of the resistancetuned amplifier circuit; Fig. 2 is a diagram -of the amplifier circuit showing in greater detail the feedback circuits and tunable bridge adjustments;

Figs. 3, 4 and 5show curves resulting from the 'adjustment of various circuit resistances;

frequency side while keeping the low frequency on broad settings; y

Fig. 8 shows the effect of extreme mismatching of tubes.

The invention applies the principle of inverse feedback to amplifiers I and 2 which consist of electron discharge tubes coupled in stages and arranged in back-to-back relation so to speak with their inputs in parallel and their outputs opposed in the primary of a transformer 3.

Each of the tunable bridges A1 and A2 in the respective amplifier networks feeds back all but a selected frequency to the input of the corresponding amplifier circuit through resistors Bl--Cl or Bz-Cz which will be described hereinafter in more detail. Each side is tuned to its own individual frequency, differing from that of the other side by an adjustable amount. Bridges A1 and A2 each contain three fixed condensers and three variable resistors. Each variable resistor contains a fixed part made up of two resistors of selected values connected in parallel to give the desired resistance within about one-half per cent. The fixed condensers of the bridge circuits are similarly built-up of measured units paralleled to give the desired capacity with accuracy.

High gain inverse feedback amplifiers tend to oscillate above and below the range of operation. This is due to phase shifts in the feedback and amplifier circuits which inevitably accompany frequency cutoff and which usually add up to degrees or more at high and low frequencies, thereby reversing the direction of the vectorial 4quantity MB (where M is the gain and B is the If the feedback is in phase with the input of the amplifier, gain is increased, sometimes to an extent causing continuous oscillation. If the feedback is opposed in phase to the input, the gain is reduced and the tendency to oscillate is likewise reduced. The gain of the amplifier is roughly the inverse of the transmission of the feedback circuit since the total gain available is high. By utilising a tuned bridge circuit which lwill not provide a feedback at 'a selected frequency the equivalent of a resonant tuned circuit is produced, and the ampliner thus eliminates the extraneous frequencies which are Vpassed by the feedback circuit. Such a system is shown in Scott Patent No. 2,173,426. It has certain advantages over conventional tuned am plifers and is useful where tron curves' of gradual cutoff are acceptable.

The present invention in one aspectv can be ,regarded as an improvement upon systems of the above type, to provide band-pass characteristics, to achieve a greater range of .adjustability, and to afford very sharp cutoff at the end or ends of a desired frequency band.

Voltages from the bridges Ai and Aa pass back through the resistors Bi-Ci, Bz-Cz to the amplifier inputs. In operation thetwo bridges are tuned to different selected frequencies, namelylthe upper and lower frequencies at which dropoff of response is desired. Although it would i seem that the ltwo outputs would cancel for a frequency midway between the two frequencies where'the outputs were equal, this does not occur because of opposite phase shifts accompanying Vthe high and low frequency resonance. Above and below the selected frequencies the outputs of both sides drop and soon have similar phase shifts and amplitudes so that low netV output is rapidly reached. Band pass characteristics are thus achieved.

The shapes of the resulting response curves depend upon the adjustments of resistors B1-B2 `and Ci-Cz as well as on thegain of amplifiers has been changedto illustrate the effect of varyv ing the band width.

In Fig. 'l three asymmetrlcaleurves are shown. These are obtained at fixed high and low frequencies by varying the sharpness of the high frequency side while keepingthe low frequency side on a broad setting.

Amplifiers I and` 2 are similar to ordinary resistance-coupled lamplifiers but use no interstage coupling condensers. The first stage is normal, while the second stage grid is connected directly to the plate of vthe rst stage. Ordinarily, the second stage grid would therefore become positive and draw a large current but lthis disastrous effect is avoided in the present invention by employing a self-biasingcathode resistor of about iifty'timesthe normal value in the second stage. The cathode is thereby maintained at a potential slightly higher than that of the preceding vplate and hencemakes the grid slightly negative with respect to its cathode. This bias is self-adjustingeven for abnormally large changes of circuit constants because a very slight deviation from normal plate current in the second stage makes alarge shift in the cathode bias in a direction tending to restore the normal current.

The gain of each side is stabilized by providing fixed inverse-feedback resistors I and 5 which couple inverse feedback from the cathode circuit of the third stagel into the cathode circuit of the first stage. This reduces the gain of the aml plifier but stabilizes the gain with respect to tube,

battery, and circuit variations by the same factor.

40 Fig. 8 shows the curves resulting from extreme of bridges A1 and A2. Figs. 3 to s ineiusive show 45 how curves of similar characteristics are automatically obtained. In each of these figures only the frequency setting knobs were adjusted to the frequencies indicated by the small arrows.

It is also desirable to provide for varying the sharpness of tuning of the two sides independently of frequency adjustments in order to change the shape of the response curves. This is done by means of resistors C1 and C2 which are ganged together for operation by asingle control means. By providing a variety of values of the resistors C1 and Cz various curve shapes are made available, including round-topped, flat-topped, double-peaked and intermediate shapes, as well as asymmetrical curves with extraordinary high attenuation just above or below the desired pass band. The amplier can also be made fiat from about 5 to 200 cycles.

In Fig. 3 is shown the curve obtained when the resistances C1 and Cz are of small value. The tuned feedback isaccordingly reduced and the resulting curves are broad and round-topped. Increasing theresistances C1 and C2 first produces. flattened curves of the shape shown in Fig. 4,` and eventually of double-peaked shape shown in Fig. 5.

Where the resistances C1 and Cz are of unequal value, the shape of thevcurves is asymmetrical but remains practically the same in character, regardless of band width, as' shown in Fig. 6. In

, change.

mismatch of tubes. The solid curve was obtained with normal tubes and'adjustments. The output and input tubes were then interchanged. Since theseftubes differ by nearly a 3 to 1 ratio in plate impedance, drastically upset bias and gain would vnormally be expected to result from such However, due to the self-biasing and fixed inverse `feedback features the peak gain dropped only 12 per cent.v The dotted curve was corrected for this drop of gain to show the true relative shapes of the response curves. The am plifier was slightly unbalanced by the change and the curve was broadened so slightly as to have negligible effect on the appearance of the recorded Waves.

Variable voltage 4divider D is provided to throw additional resistance from one side of the circuitto the other as required to equalize or balance the gains of the two sides. Each of the adjustments is almost entirely independent of the others with the result that tuning is relatively simple. 'I'here are only three adjustments which are normally used, two ofwhich are calibrated directly in logarithmic frequency steps and are set to the upper and lower frequencies at which drop-ofi! of response is desired, while the third control selects the desired `shape of response curve. The response curves can, by manipulation of these controls be shifted to any part of the frequency spectrum without change of shape.

, The new amplifier is exceptionally versatile; its use is not confined to seismograph prospecting. but it is susceptible of broader adaptation. It

can duplicate the characteristics of any ordinary' seismograph amplifier and at the same time procurves here represented. thelcw frequencyv drop-olf of response is desired. said bridges and connections providing inverse feedback of ail other frequencies, whereby extraneous frequencies are eliminated, the outputs of said amplifier circuits being in opposition, resulting in a band-pass of desired characteristics within the range of the selected upper and lower frequencies.

6. An amplifier comprisinga pair of amplier circuits, a plurality of coupled electron discharge comprising a pair amplifier circuits, means in said circuits for passing selected adjacent overlapping bands of frequencies of similar band width and suppressing undesired frequencies,

said circuits having-'their inputs in parallel and their outputs in opposition to produce a bandpass of desired characteristics.

2. An amplifier of band-pass characteristics .comprising a pair of resistance-tuned amplier circuits, resistance tuned bridges in said circuits for passing selected adjacent overlapping bands of frequencies of `similar band width and suppressing undesired frequencies, said circuits having their inputs in parallel and their outputs in opposition to produce a band-pass of desired characteristics.

3.` An amplier of band-pass characteristics 'comprising a plurality of coupled electron discharge tubes in each of a pair of amplifier circuits, means in each circuit to feed back inversely all but a selected frequencyl to be passed through such circuit, thereby to eliminate extraneous frequencies the said selected frequencies being overlapping bands of similar width, said circuits having their inputs in parallel and their outputs in opposition to produce a band-pass of desired characteristics.

4. An amplifier of band-pass characteristics comprising a pair of amplifier circuits, a plurality ofcoupled electron discharge tubes in each such circuit, tuned bridges in said circuits with connections for feedback of all but selected frequencies to the inputs of the amplifiers, said selected frequencies being of similar band-width and overlapping, the outputs of said circuits being in opposition to produce a band-pass within the range of the selected upper and lower frequencies.

5. An amplifier comprising a pair of resistancetuned circuits, the inputs of which are in parallel, a plurality of coupled electron discharge tubes in each circuit, a bridge in each circuit, connections from each bridge to the grid of the first of said coupled electron discharge tubes, means for tuning the bridges to desired upper and lower frequencies, respectively, at which tubes in each circuit, bridges in each circuit having connections for inverse feedback to the first stage of said coupled electron tubes, means for selectively tuning said bridges to two distinct desired frequencies so that each bridge will feed back al1 but o-ne of said desired frequencies, whereby undesired frequencies will be suppressed in each circuit, a variable resistance in each -amplifler circuit arranged to adjust the sharpness of tuning of said circuit, means for simultaneously adjusting said resistances when tuning said circuits; the inputs of said circuits being in parallel and their outputs in opposition, resulting in a band-pass of desired characteristics within the range of the selected upper and lower frequencies.

7. The combination of claim 1 wherein the amplifier circuits are adjusted in unison by ganging of the controls so that the breadth of resonance of each tuner is proportional to the frequency interval between them.

8. The combination of claim 3 wherein iixed feedback is provided in the cathode circuits of the electron discharge tubes and the feedback from plate to grid circuits is tuned to give ba.- anced curves with minimum interaction between the various feedback paths.

9. The combination of claim 3 wherein the gain of each amplifier is stabilized by a fixed inverse feedback resistor coupling inverse feedback from the cathode circuit of the third stage into the cathode circuit of' the first stage.

l0. An amplifier of band-pass characteristics comprising a pair of amplifier networks having their inputs in parallel and their outputs in opposition', tunable bridges in said respective networks for inverse feedback to the input of each amplifier of all but an individual selected frequency whereby undesired frequencies are eliminated from the output of each amplifier, variable resistances in said respective networks for changing the sharpness of tuning of each network, the arrangement being such that the frequency response function of the amplifier is adinstable both as to range of frequencies passed and suppressed, and, independently of such adjustment, as to the relative sensitivity for the several frequencies.

GARY MUFFLY. 

